1. Field of the Invention
This invention relates to an improved method for forming plastic packages for electronic semiconductor devices, which is particularly useful in the instance of thin packages including a supporting and electric connection metal frame and a plastic body. The invention also relates to a package with improved characteristics. The invention is specifically directed to the use of a mold of improved shape for forming the plastic casing.
2. Description of the Prior Art
As is well known, integrated electronic semiconductor devices, comprising an integrated circuit, are formed on so-called "dies" of a semiconductor maternal which are mounted integrally to electric interconnection structures enclosed in a body of a synthetic plastic material serving a protective function.
In the specific field of this invention, such structures typically comprise a support, usually a thin metal foil (leadframe), the central portion whereof, usually depressed, accommodates the die, and which has terminal leads. The die is, across one of its large surfaces, in contact with the leadframe, the integrated circuit being exposed on the opposite die surface in electric contact with the leads. A plastic body fully encloses the integrated device, and in part the leadframe on which this is mounted. The terminal parts of the leads, or pins, are located on the body exterior and function as electric connections for connecting the device to an external circuit.
Within the scope of this invention, of special concern are reduced-thickness packages, having body length and width which are much greater than its thickness. These are commonly referred to as thin packages, and are supplied to the field in different types and for different applications. Usually, they are employed for devices which require no large power loss, that is, devices which are designed for operating on comparatively small currents. For a better understanding of the aspects of this invention, it pays to briefly review the usual steps of a conventional package forming process.
Typically, the dies are first placed on a certain number of leadframes formed on a single metal strip, and the electric connections to the leads are established using thin metal wires. The strip carrying the dies is then introduced into a mold having cavities corresponding to the individual devices, and a molten, electrically insulating material is injected at a high temperature to form the monolithic plastic body of the package. This material is usually a synthetic resin, such as an epoxy resin. The molding step includes a number of stages at which the temperature is gradually changed to avoid any risks of breaking the semiconductor material or, in any case, reducing the device reliability. On this account, moreover, the leadframe is heated before the molten material is poured in. After injecting the resin, that is after the molding in the proper sense of the word, and following a first cooling step with consequent partial polymerization, the resin is subjected to thermodynamic curing processes. The latter will improve the material properties by promoting thorough polymerization and stretching the long molecules which compose the polymeric material of the resin. The batch of packages thus formed is then taken out of the mold.
A known mold for injecting the resin is depicted in FIG. 1, which shows an exploded vertical section view taken through a single cavity, i.e. a single package, of the batch. A leadframe 1 having a semiconductor die mounted thereon is placed into the mold cavity with the ends of the terminal leads protruding outside the cavity. The mold comprises, in the embodiment shown, two parts: a lower half-mold or half-shell die 3 and an upper half-mold or half-shell 4, each having a corresponding hollow therein. The two half molds are disposed with their hollows facing each other to define a cavity whereinto the resin is then injected through an inlet port 5 formed in the mold itself.
In accordance with the prior art, the large parametric walls delimiting the cavity (bottom wall 6 and top wall 7) are made flat in order to ideally produce a body having flat parallel top and bottom surfaces. Actually, despite the precautions taken to avoid exposure of the whole device to abrupt temperature changes brought about by the aforementioned thermal treatments, the package leaves the molding and drawing steps with a bending or so-called warpage deformation in it. The large surfaces of the resultant body are not flat, but with the central region of each surface that is sunk below or raised above the plane described by the corresponding edges.
This behavior is due to the fact that the different materials which make up the complete structure of the package, i.e. the metal leadframe, the semiconductor die, and the resin, have different thermal properties (different expansion and shrinkage coefficients). In addition, the asymmetrical distribution about the horizontal axis A--A, as shown in FIG. 1, of the materials, i.e. the leadframe 1 and the semiconductor die 2, inside the package body, generates internal stresses which contribute toward a warped package.
In other words, the ideal plane containing the leadframe--represented in section by the axis A--A--is subjected to compression in one of the directions shown by the arrows C1 and C2 in FIG. 1. This compression causes a deflection to occur in said plane, and hence in the body as a whole. The actual shape of the package at room temperature is that shown in FIGS. 2 and 3, which are sectional views taken along either of the large dimensions of the package (vertical sections). The amount of the deformation undergone has been exaggerated in the drawing figures for clarity. Viewed in cross-section, the body outline is basically that of a parallelogram with two long sides showing a concavity in the same direction.
Shown in FIG. 2 is a package with a concavity upwards, whereas in FIG. 3 the package has a concavity downwards. The direction of the bending is individual to each package and not fully predictable, although a deformation of the kind shown in FIG. 2 is more likely to occur with an internal structure of the package having the distribution of the materials shown in the figures.
As shown in both FIGS. 2 and 3, the device as a whole, i.e. both the leadframe 1 (together with the semiconductor die 2 mounted on it) and the plastic body, generally denoted by 8, is bent at this intermediate stage of its forming process. In particular, the body departs from the ideal shape, with flat top and bottom surfaces, by a degree of depression or elevation, with respect to a plane described by the edges of each of these surfaces, of a plane tangent the corresponding surface in its central region.
This depression (or elevation) is known as warpage, designated W in FIGS. 2 and 3. With a body illustratively measuring about 18-20 mm in width and length and about 1 mm in thickness, as would be typical of current thin packages, warpage my amount to a few tens of microns. Although the amount of warpage deformation is proportionally small compared to the size of the body, still it can grow into a serious problem during the following steps for completing the forming of the device, and does lower the reliability of the finished device.
After removing the packages from the mold, according to the prior art, the individual packages are separated from one another (the so-called singulation step) by cutting up the metal strip on which the batch of leadframes has been formed. Thereafter, for each package, that portion of the leads which has been allowed to protrude out of the package body is bent to create pins (the so-called lead forming step) that will enable the electric connection of the device for a particular application, usually by welding the pins to a printed circuit.
During both of these operations, and as shown schematically in FIG. 4, the plastic body 8 of the package is held between a bottom clamping surface 9 and a top clamping surface 10. Also, its position is set such that it is centered between side retaining means, not shown in FIG. 4. The effective clamping distance H, in practice the gap between the two clamping surfaces 9 and 10, must be preset in the equipment used for these manufacturing steps. To take account of the actual shape of the body, and ensure proper clamping thereof, the distance H is set for an average amount of bending deformation in a package. This is calculated as the sum of the ideal (i.e., undeformed) height of the body--as determined by the thickness of the mold cavity--plus the average deformation thereof. Indicated by arrows in FIG. 4 are the forces F which are applied to the package, specifically at the points of contact between the plastic body 8 and the clamping surfaces 9 and 10.
Despite the equipment having been preset, problems may be encountered during these final steps of the device forming process. In the instance of packages which exit the mold in a markedly bent state, or, at any rate, with an above-average amount of curvature, the unevenly distributed stresses to which the body would be subjected (as shown in FIG. 4 by the arrows F) are quite large, and the package would receive an appreciable bending moment.
Accordingly, there exists a risk of straining the structure of the package or, in the extreme, of breaking the semiconductor die or starting a delamination between internal parts of the body, i.e. of the semiconductor die parting from the leadframe. On the other hand, a slightly deformed--less than a predetermined average amount--package cannot be held securely between the clamping surfaces nor correctly centered.
The likely outcome of this are processing difficulties and/or poor results, which would result in an increased number of rejections. As regards the structural and functional characteristics of the finished device, and hence its reliability, a heavy warpage deformation can result in failure to meet the specifications for a particular application of the device. Major factors in the context of this invention are the overall height of the device (as measured to the outside of pins and plastic body), and the so-called standoff, that is the relative height of the lowermost point on the body and of a plane described by the pin base.
FIGS. 5 and 6 show two prior art devices upon completion of their forming process. The device of FIG. 5 has been obtained by bending the pins 1a and 1b to opposite directions with respect to the bending undergone by the body 8. In the ideal instance of a body developing no deformation, the device standoff would be exactly equal to the constant relative height of the flat bottom surface of the body above a rest plane described by the pin base.
The introduction of warpage diminishes, as shown in FIG. 5, the actual standoff value, denoted by S, with respect to the ideal value, denoted by S.sub.ideale. In fact, the central region of the bottom surface 11 of the plastic body 8 is sunk with respect to the edges, due to the bending deformation. FIG. 5 illustrates an extreme case where this central region is also sunk with respect to the pin base. In all cases, the pins may fail to adhere tightly to the surface on which the package rests. A typical amount of standoff would be in the range of 0.1 (0.05 mm. Thus, the admissible error may be approximately the same magnitude as the warpage deformation.
The amount of standoff may prove inadequate to ensure electric contact of the device with a printed circuit upon welding, or in any case, may be less than a minimum provided for by its specifications.
FIG. 6 shows a package wherein the pins have been bent in the same direction as the curvature of the body. Here, the effect of the body warpage deformation is that of increasing the overall height of the device. Since in this instance the top surface 12 of the body 8 is bent, with a raised central region, the overall height of the device, as denoted by T in FIG. 6, is given by the ideal overall height of the device, denoted by T.sub.ideale in the same Figure, plus the amount of warpage W.
As shown, the ideal overall height T.sub.ideale can be obtained as the sum of the constant ideal thickness B of the plastic body 8 plus the amount of standoff S.sub.ideale, which in this case can be considered to be substantially the same as the actual standoff. Where the effect of the deformation is particularly evident, the packages may deviate from the specifications set for the device application, thereby increasing the rate of production rejections.